Diogo Jota and Cody Gakpo both struck twice, with Mohamed Salah also scoring. West Hamhad gone ahead through a Jarell Quansah own goal.
Slot later said: "You already start to laugh because you're thinking for two months I only ask about new players! I constantly told you guys how many quality players we already had. I think you all knew and that's also what we showed in the last weeks and today again that we have many quality players.
"What pleased me most was that even if a lot of new players come in they don't come in and try to have a good individual performance. They try to work really hard for the team. There was a lot of desire not to concede and as a result of that we had some good individual performances as well."
On Jota and Gakpo, he continued: "I think Diogo deserved it because he played a lot of good games for us already. If I'm correct he was only on the scoresheet in the first game against Ipswich. He's been important for us in every single game he played but a striker also wants to score goals.
"I'm pleased for him that he scored two today although he played in a No.10 position. He scored two really good goals and Cody I think it was 10, 15 or 20 minutes before the end where he cleared the ball in his own 18-yard box and that is also what he brings to our game apart from scoring two.
"If I see him in training and the way he hits the ball in and around the 18-yard box it is with incredible speed. So, I'm not surprised to see him scoring like this. Someone just told me he scored a similar one last season in this cup tie against West Ham. So, I'm not surprised to see him score; not in the way he did, but it's not only about them scoring, it's also about them defending in their own 18-yard box."